PKC Activators in Alzheimer's Disease, Stroke, and Depressive Disorders
Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the progressive decline of memory and cognitive functions. Dementia associated with AD is referred to as senile dementia of the Alzheimer's type (SDAT) in usage with Alzheimer's disease. AD is characterized clinically by progressive loss of memory, cognition, reasoning, judgment, and emotional stability that gradually leads to profound mental deterioration and ultimately, death. Although there are many hypotheses for the possible mechanisms of AD, one central theory is that the excessive formation and accumulation of toxic beta-amyloid (Aβ) peptides either directly or indirectly affects a variety of cellular events and leads to neuronal damage and cell death. Selkoe, Neuron. 1991; 6(4):487-98 1991; Selkoe, J Clin Invest. 2002; 110(10):1375-81.
AD is a progressive disorder with a mean duration of around 8-15 years between onset of clinical symptoms and death. AD is believed to represent the seventh most common medical cause of death and affects about 5 million people in the United States. The prevalence is expected to reach 7.7 million by 2030. About 1 in 8 people over the age of 65, 13% of this population, have AD (Alzheimer's Association 2008 Alzheimer's Disease Facts and Figures). AD currently affects about 15 million people world-wide (including all races and ethnic groups) and owing to the relative increase of elderly people in the population its prevalence is likely to increase over the next two to three decades. AD is at present incurable.
Protein kinase C (PKC) is one of the largest gene families of protein kinase. Several PKC isozymes are expressed in the brain, including PKC, PKCβ1, PKCβII, PKCδ, PKCε, and PKCγ. PKC is primarily a cytosolic protein, but with stimulation it translocates to the membrane. PKC has been shown to be involved in numerous biochemical processes relevant to Alzheimer's disease. PKC activation also has a crucial role in learning and memory enhancement and PKC activators have been shown to increase memory and learning. Sun and Alkon, Eur J. Pharmacol. 2005; 512:43-51; Alkon et al., Proc Natl Acad Sci USA. 2005; 102:16432-16437. PKC activation also has been shown to induce synaptogenesis in rat hippocampus, suggesting the potential of PKC-mediated antiapoptosis and synaptogenesis during conditions of neurodegeneration. Sun and Alkon, Proc Natl Acad Sci USA. 2008; 105(36): 13620-13625. Postischemic/hypoxic treatment with bryostatin-1, a PKC activator, effectively rescued ischemia-induced deficits in synaptogenesis, neurotrophic activity, and spatial learning and memory. Sun and Alkon, Proc Natl Acad Sci USA. 2008. This effect is accompanied by increases in levels of synaptic proteins spiniophilin and synaptophysin and structural changes in synaptic morphology. Hongpaisan and Alkon, Proc Natl Acad Sci USA. 2007; 104:19571-19576. Bryostatin-induced synaptogenesis for long-term associative memory is also regulated by PKC activation. Hongpaisan and Alkon, PNAS 2007. PKC also activates neurotrophin production. Neurotrophins, particularly brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), are key growth factors that initiate repair and regrowth of damaged neurons and synapses. Activation of some PKC isoforms, particularly PKCε and PKCα, protect against neurological injury, most likely by upregulating the production of neurotrophins. Weinreb et al., FASEB Journal. 2004; 18:1471-1473). PKC activators are also reported to induce expression of tyrosine hydroxylase and induce neuronal survival and neurite outgrowth. Du and Iacovitti, J. Neurochem. 1997; 68: 564-69; Hongpaisan and Alkon, PNAS 2007; Lallemend et al., J. Cell Sci. 2005; 118: 4511-25.
AD is also characterized by tau hyperphosphorylation. Tau is expressed mainly in the brain, where it regulates the orientation and stability of microtubules in neurons, astrocytes and oligodendrocytes. In AD, normal soluble tau is transformed into insoluble paired helical filaments. This is linked to the post-translational change in tau, primarily the hyperphosphorylation of tau by a number of protein kinases. Studies have shown that synthetic Aβ promotes tau phosphorylation through activation of glycogen synthase kinase-3 GSK-3. Wang et al., Journal of Neurochemistry. 2006; 98(4): 1167-1175. Activation of PKC has been shown to protects rat primary hippocampal neurons from Aβ-mediated neurotoxicity, through inhibition of GSK-3β. Garrido et al., FASEB J. 2002: 1982.
PKC also activates TNF-alpha converting enzyme (TACE, also known as ADAM17), which is an enzyme that is involved in the proteolytic conversion of membrane-bound amyloid precursor protein (APP) to its non-pathogenic soluble form, known as soluble APP-alpha or sAPPα. Alkon et al., Trends in Pharmacological Sciences. 2007; 28(2): 51-60; Hurtado et al., Neuropharmacology. 2001; 40(8): 1094-1102. These sAPPα-producing enzymes are referred to generically as alpha-secretases. Activation of TACE by PKC also reduces cellular levels of pathogenic Aβ, which is produced by cleavage of APP by the beta-secretase enzyme (BACE). This is likely due to the fact that the TACE cleavage site is within the Aβ domain of APP. Overexpression of PKCε has been shown to selectively increase the activity of endothelin-converting enzyme (ECE), which degrades Aβ. Choi et al., Proc. Natl. Acad. Sci. USA. 2006; 103(21): 8215-8220. In addition, sub-nanomolar concentrations of bryostatin and a potent synthetic analog (picolog), both PKC activators, were found to cause stimulation of non-amyloidogenic pathways by increasing TACE and thus lowering the amount of toxic Aβ produced. Khan et al., Proc. Natl. Acad. Sci. USA. 2009; 34(2):332-9.
Reduction of Aβ levels is a major therapeutic goal in Alzheimer's disease. It has been speculated that inhibition of Aβ formation by PKC activators may be caused by competition of TACE and BACE for their common substrate, APP.
The strategy of PKC-mediated activation of α-secretases has the advantage of three parallel beneficial consequences in AD: increasing production of sAPP-α and reducing Aβ, enhancing memory via PKC-mediated phosphorylation of downstream substrates, and decreasing phosphorylation of tau through inhibition of GSK-3β.
AD patients already have reduced levels of PKCα/ε-mediated phosphorylation of Erk1/2, a major downstream substrate of PKC. Khan and Alkon, Proc Natl Acad Sci USA. 2006; 103:13203-13207. In addition, Aβ application to normal fibroblasts reduces PKC activity because Aβ directly down-regulates PKC α/ε. PKC activators, especially those specific for PKC α/ε, would counteract the effect of Aβ and thereby reverse or prevent the Aβ-induced changes.
Stroke is a leading cause of disability and death in the United States, yet limited therapeutic options exist. Several PKC isoforms have been shown to have a central role in mediating ischemic and reperfusion damage following stroke. Studies with experimental stroke models, mouse genetics, and selective peptide inhibitors and activators have demonstrated that PKCε is involved in induction of ischemic tolerance and prevents damage, while PKCδ and γ are implicated in injury. Takayoshi et al., Stroke. 2007; 38(2):375-380; and Bright et al., Stroke. 2005; 36: 2781. One possible mechanisms for PKCε's protective ischemic effect is that PKCε maintaining mitochondrial function via ERK activity and by mediating adenosine-induced mitochondrial ATP-sensitive potassium channels. Another potential mechanism is that PKCε elicits a neuroprotective effect via COX-2 induction. Kim et al., Neuroscience. 2007; 145(3): 931-941. Prostaglandin E2 (PGE2), the product of COX-2 activity, leads to neuroprotection in cerebral ischemia. As mentioned above, postischemic/hypoxic treatment with bryostatin-1, a PKC activator, effectively rescued ischemia-induced deficits in synaptogenesis, neurotrophic activity, and spatial learning and memory. Sun and Alkon, Proc Natl Acad Sci USA. 2008; 105(36): 13620-13625.
Circulating Aβ protein has been shown to be elevated in patients with acute ischemic stroke Circulating Aβ1-40 level was markedly elevated in ischemic stroke patients, as compared to controls. Lee et al., Journal of Neural Transmission. 2005; 112(10): 1371-79. A strong positive association between progressively accumulating vascular Aβ and augmentations in arteriole and frontal cortex wall thickness AD patients also has been shown, suggesting that the continually progressing Aβ-associated angiopathy, at the arteriolar level, harms the contractile apparatus and cerebral blood flow autoregulation, thereby making the downstream capillaries vulnerable to damage. Stopa et al., Stroke. 2008; 39:814.
In addition, some forms of stroke are caused by Aβ, such as those associated with cerebral amyloid angiopathy, also known as congophilic amyloid angiopathy (CAA). This disorder is a form of angiopathy in which the same Aβ deposits as found in AD accumulate in the walls of the leptomeninges and superficial cerebral cortical blood vessels of the brain. Amyloid deposition predisposes these blood vessel to failure, increasing the risk of a hemorrhagic stroke. CAA is also associated with transient ischemic attacks, subarachnoid hemorrhage, Down syndrome, post irradiation necrosis, multiple sclerosis, leucoencephalopathy, spongiform encephalopathy, and dementia pugilistica.
Evidence suggests that PKCα and ε are the most important PKC isoforms in eliciting the aforementioned beneficial effects in AD, stroke, and depressive disorders. Antisense inhibition of PKCα has been shown to block secretion of sAPPα, while PKCε is the isozyme that most effectively suppresses Aβ production. Racci et al., Mol. Psychiatry. 2003; 8:209-216; and Zhu et al., Biochem. Biophys. Res. Commun. 2001; 285: 997-1006. Thus, isoform specific PKC activators are highly desirable as potential anti-Alzheimer's drugs. Specific activators are preferable to compounds such as bryostatin that show less specificity because non-specific activation of PKCδ or β could produce undesirable side effects.
Moreover, PKCε is also expressed at very low levels in all normal tissues except for brain. Mischak et al., J. Biol. Chem. 1993; 268: 6090-6096; Van Kolen et al., J. Neurochem. 2008; 104:1-13. The high abundance of PKCe in presynaptic nerve fibers suggest a role in neurite outgrowth or neurotransmitter release. Shirai et al., FEBS J. 2008; 275: 3988-3994). Therefore, effects of specific PKCε activators would be largely restricted to brain, and unlikely to produce unwanted peripheral side effects.
PUFAs as PKC Activators
Some PUFAs, such as arachidonic acid (see FIG. 1), have been known for many years to be natural activators of PKC. Docosahexaenoic acid (DHA) is also a known activator of PKC and has recently been shown to slow the accumulation of Aβ and tau proteins associated with the brain-clogging plaques and tangles implicated in AD. Sahlin et al., Eur J Neurosci. 2007; 26(4):882-9.
Kanno et al. described effect of 8-[2-(2-pentyl-cyclopropylmethyl)-cyclopropyl]-octanoic acid (DCP-LA), a newly synthesized linoleic acid derivative with cyclopropane rings instead of cis-double bonds, on protein kinase C (PKC) activity. Journal of Lipid Research. 2007; 47: 1146-1156. DCP-LA activated PKCε, with a greater than 7-fold potency over other PKC isozymes. This indicates that DCP-LA is highly specific for PKCε. This compound also facilitated hippocampal synaptic transmission by enhancing activity of presynaptic acetylcholine receptors on the glutamatergic terminals or neurons. However, DCP-LA requires relatively high concentrations to produce its maximal effect.
WO 2002/50113 to Nishizaki et al., discloses carboxylic acid compounds and their corresponding salts having cyclopropane rings for LTP-like potentiation of synaptic transmission or for use as a cognition-enhancing drug or a drug to treat dementia. Their synthetic examples disclose preparation of esters but their experimental results teach the use of free acids. The reason is that the carboxylic acid group of the fatty acid starting material would react with the diethylzinc used in the Simmons-Smith reaction. The methyl ester acts as a protecting group and may be cleaved off by hydrolysis or allowed to remain as needed.
The caveats with the prior art finding include the necessity of administering high concentrations of to achieve the foregoing effects, non-specific activation of PKC isoforms, or rapid metabolism and sequestration of unmodified PUFAs into fat tissues and other organs where they are incorporated into triglycerides and chylomicrons. J. Pharmacobiodyn. 1988; 11(4):251-61. In addition use of unmodified PUFAs would have a myriad of adverse side effects. For example, arachidonic acid is a biochemical precursor to prostaglandins, thromboxanes, and leukotrienes, which have potent pro-inflammatory effects. This would be undesirable for treatment of Alzheimer's disease where the pathology likely involves inflammation. Other essential fatty acids may also possess a multitude of other biological effects, including enhancement of nitric oxide signaling, anti-inflammatory effects, and inhibition of HMG-CoA reductase, which would interfere with cholesterol biosynthesis.
Because of the limited existing options for treating both AD and stroke, new therapeutics that can selectively activate only the PKC isoforms that elicit neuroprotection are needed.
PUFAs and MUFAs and Disease
A growing number of studies have suggested that omega-3 PUFAs can be beneficial for other mood disturbance disorders such as clinical depression, bipolar disorder, personality disorders, schizophrenia, and attention deficit disorders. Ross et al., Lipids Health Dis. 2007; 18; 6:21. There is an abundance of evidence linking omega-3 fatty acids, particularly docosahexaenoic and eicosapentaenoic acids, and a healthy balance of omega-3 to omega-6 fatty acids, to lowering the risk of depression. Logan et al., Lipids Health Dis. 2004; 3: 25. Levels of omega-3 fatty acids were found to be measurably low and the ratio of omega-6 to omega-3 fatty acids were particularly high in a clinical study of patients hospitalized for depression. A recent study demonstrated that there was a selective deficit in docosahexaenoic in the orbitofrontal cortex of patients with major depressive disorder. McNamara et al. Biol Psychiatry. 2007; 62(1):17-24. Several studies have also shown that subjects with bipolar disorder have lower levels omega-3 fatty acids. In several recent studies, omega-3 fatty acids were shown to be more effective than placebo for depression in both adults and children with bipolar depression. Osher and Belmaker, CNS Neurosci Ther. 2009; 15 (2):128-33; Turnbull et al., Arch Psychiatr Nurs. 2008; 22(5):305-11.
Extensive research also indicates that omega-3 fatty acids reduce inflammation and help prevent risk factors associated with chronic diseases such as heart disease, cancer, inflammatory bowel disease and rheumatoid arthritis. Calder et al., Biofactors. 2009; 35(3):266-72; Psota et al., Am J Cardiol. 2006; 98(4A):3i-18i; Wendel et al., Anticancer Agents Med. Chem. 2009; 9(4):457-70.
Monounsaturated fatty acids also have been shown to be beneficial in disorders. There is good scientific support for MUFA diets as an alternative to low-fat diets for medical nutrition therapy in Type 2 diabetes. Ros, American Journal of Clinical Nutrition. 2003; 78(3): 617S-625S. High-monounsaturated fatty acid diets lower both plasma cholesterol and triacylglycerol concentrations. Kris-Etherton et al., Am J Clin Nutr. 1999 December; 70(6):1009-15.
The present invention provides a method for activating PKCε using certain derivatives of polyunsaturated fatty acids (PUFA) or monounsaturated fatty acids (MUFA). These compounds activate PKCε at nanomolar concentrations which makes them excellent candidates thr the treatment of AD, stroke, and other neurological diseases in which PKCε is neuroprotective.